Optically pumped, vertically emitting semiconductor laser

ABSTRACT

An optically pumped vertically emitting semiconductor laser having a highly reflective reflector layer ( 10 ), and a radiation-emitting active layer sequence ( 14 ) arranged on the reflector layer, in which, during operation, the reflector layer ( 10 ), together with an external mirror ( 20 ), forms a laser resonator. A heat sink ( 12 ), transparent to the emitted radiations is arranged within the laser resonator ( 10, 20 ) and in thermal contact with the active layer sequence ( 14 ), said heat sink being formed from a material having a higher thermal conductivity than the materials of the active layer sequence ( 14 ).

The invention relates to an optically pumped vertically emittingsemiconductor laser having a highly reflective reflector layer, and aradiation-emitting active layer sequence arranged on the reflectorlayer, in which, during operation, the reflector layer, together with anexternal mirror, forms a laser resonator.

Such semiconductor lasers are disclosed for example in the article M.Kuznetsov et al., “Design and Characteristics of High-Power (>0.5 W CW)Diode-Pumped Vertical-External-Cavity Surface-Emitting SemiconductorLasers with Circular TEM₀₀ Beams”, IEEE J. Selected Topics QuantumElectron., Vol. 5, No. 3, pp. 561-573, 1999.

These VECSELs enable a high continuous wave radiation power into thewatts range with a good beam quality. However, during high-powercontinuous wave operation, the problem arises of efficiently dissipatingfrom the component the heat loss generated for example by nonradiativerecombinations.

For this purpose, the optically pumped VECSEL described by Kuznetsov etal. is mounted on a diamond heat sink having a high thermalconductivity. Since the growth substrate substantially contributes tothe thermal impedance of the component, it is preferably removed and theVECSEL is applied to the heat sink directly with the mirror layers.

However, this procedure necessitates an upside-down mounting of thecomponent. In addition, when the growth substrate is removed, there isthe risk of the component being fractured.

This is the starting point of the invention. The invention, as it ischaracterized in the claims, is based on the object of specifying avertically emitting semiconductor laser of the generic type which avoidsthe disadvantages of the prior art and which, in particular, can bereliably operated even at high continuous wave powers.

This object is achieved according to the invention by means of thevertically emitting semiconductor laser according to claim 1 and thevertically emitting semiconductor laser having a radiation-emittingactive layer sequence according to claim 10. Preferred refinementsemerge from the subclaims.

According to the invention, in a vertically emitting semiconductor laserof the type mentioned in the introduction, a heat sink transparent tothe emitted radiation is arranged within the laser resonator and inthermal contact with the active layer sequence, said heat sink beingformed from a material having a higher thermal conductivity than thematerials of the active layer sequence.

The invention is based on the observation that the reflector layer,which is usually formed by a distributed Bragg reflector, that is to sayan alternating sequence of layers having a high and low refractive indexwith a thickness of λ/4n, constitutes a significant obstacle to thedissipation of heat. Proceeding from this, the invention is based on theconcept, for the heat dissipation, of bypassing the reflector layer orof using heat spreading to increase the area over which the reflectorlayer has to dissipate the heat.

This aim is achieved by means of a heat sink which is arranged withinthe resonator and is in thermal contact with the active layer sequence,the thermal conductivity of which is higher than that of the materialsof the active layer.

In this case, the configuration of the heat sink and of the reflectorlayer makes it possible for the dissipated heat flow to practicallycompletely bypass the reflector layer.

As an alternative, it is possible to fashion the heat sink such that thedissipated heat flow is spread and flows through the reflector layerover a larger area. Since the thermal resistance is inverselyproportional to the cross-sectional area of the heat flow, a reductionof the thermal resistance is achieved.

Preferably, the heat sink is formed from readily thermally conductivematerials which are transparent in the red and/or near infrared, such asSiC, BN or diamond.

In an advantageous refinement, the heat sink is formed by a layer havinga thickness of at least 0.2 times the diameter of the active zone. Itmay have, for example, a thickness of between 3 and 50 μm, preferablybetween 5 and 30 μm, particularly preferably between 10 and 20 μm.

The heat sink is expediently arranged between the active layer sequenceand the reflector layer. The heat flow is then expanded by the heat sinkbetween the active layer sequence and the reflector layer and thethermal resistance of the reflector layer is thus reduced.

In this case, the heat sink may be in direct contact with the activelayer sequence, or a partly reflective Bragg reflector may be arrangedbetween the heat sink and the active layer sequence. This last isexpedient in particular when the reflectivity of the reflector layerbehind the heat sink alone is insufficient and the reflector layercomprises gold, for example.

In preferred refinements, the reflector layer is formed by a distributedBragg reflector containing layers alternately having a high and lowrefractive index with a thickness of essentially λ/4n. In this case, λdesignates the wavelength of the radiation emitted by the active zoneand n designates the refractive indices of the corresponding layer.

The individual reflections at the layers are then superposedconstructively, thereby achieving a high total reflectivity of greaterthan 99%, preferably greater than 99.5%, particularly preferably greaterthan 99.9%. The reflectivity R of a Bragg mirror comprising m layerpairs having refractive indices n1 and n2, respectively, is given by$R = \frac{1 - \left( {{n1}/{n2}} \right)^{m}}{1 + \left( {{n1}/{n2}} \right)^{m}}$Thus, a high reflectivity requires a large index contrast n1/n2 and asufficient number of layer pairs. Moreover, for the validity of theaforementioned formula, the layer thickness has to be met sufficientlyprecisely.

In the context of the invention, the Bragg mirror may either be grownepitaxially, or dielectric layers are deposited. While thelast-mentioned method yields high index contrasts, the requiredthicknesses can be met precisely only with difficulty. Moreover,dielectrics have only a low thermal conductivity.

With the use of epitaxial semiconductor layers, the required thicknessescan be complied with very precisely and the thermal conductivity issomewhat higher than that of the dielectrics. On the other hand, it isnecessary that the materials do not absorb at the emission wavelengthand that the lattice constant must be identical for all the layers.Other lattice constants produce crystal defects which, besides a poorermirror, prevent the growth of a high-quality active layer sequence.Consequently, the use of epitaxial semiconductor layers entailslimitation to a few materials.

In the infrared, by way of example, it is possible to use the AlGaAsmaterial system which is grown in lattice-matched fashion on GaAssubstrates: in the infrared, the refractive indices are about 3.0 (AlAs)and 3.5 (GaAs) and the lattice constant is virtually independent of thealuminum content.

For red-emitting VECSELs, the Bragg mirror may likewise comprise theAlGaAs system. Owing to the shorter wavelength, however, GaAs absorbsand must therefore be replaced by AlGaAs with at least 60% aluminum. Themixed semiconductor in turn has a poorer thermal conductivity than thebinary semiconductors AlAs and GaAs. Moreover, the index contrast isreduced, which has to be compensated for by a higher number of mirrorpairs.

The thermal conductivity of the Bragg mirror has proved to be verycritical: although for example GaAs and AlAs have thermal conductivitiesof 46 and 91 W/(m K), two successive thin layers made of differentmaterials have a poorer thermal conductivity since, owing to the changein the crystal, the lattice vibrations (phonons) do not correspond.Therefore, the lattice vibrations are partly reflected at theseinterfaces and the entire thermal resistance is thus increased owing tothis heat transfer resistance.

In this case, a real Bragg mirror typically comprises 30 or 40 layerpairs, that is to say 60 or 80 interfaces, respectively, which in eachcase contribute to the thermal resistance.

The heat sink or a further heat sink may also be arranged on that sideof the active layer sequence which is remote from the reflector layer,so that the heat is predominantly conducted not directly via thereflector layer but into the heat sink.

Ultimately, the heat can, of course, flow from the heat sink again viathe reflector layer, but then, owing to the heat spreading, over asignificantly larger area and thus a lower resistance.

As an alternative, heat sink and reflector layer may be arranged suchthat the heat flows away outside the reflector layer from the heat sinkinto a substrate or some other larger heat reservoir.

This may be done for example by the Bragg mirror being formed only in acentral region of the component, so that the heat can be dissipated inan outer region without having to flow through the Bragg mirror.

In a different configuration, the optically pumped vertically emittingsemiconductor laser has a radiation-emitting active layer sequence andtwo external mirrors, which form a vertical resonator. A heat sinktransparent to the emitted radiation is arranged within the verticalresonator and in thermal contact with the active layer sequence, saidheat sink being formed from a material having a higher thermalconductivity than the materials of the active layer sequence.

The arrangement according to the invention is particularly preferablyused in optically pumped vertically emitting semiconductor lasers inwhich the pump source is arranged laterally with respect to theoptically pumped vertically emitting semiconductor laser and ismonolithically integrated together with the latter on a commonsubstrate. In this case, the pump source preferably has an edge emittinglaser structure or a plurality of edge emitting laser structures whichis or are grown by means of epitaxy before or after the epitaxialfabrication of the structure of the optically pumpable verticallyemitting semiconductor laser on the common substrate. Such laserarrangements are disclosed in DE 100 26 734 and, therefore, are notexplained in any further detail at this juncture.

In all the embodiments mentioned, the radiation-emitting active layersequence may comprise a GaAs- or InP-based semiconductor material, inparticular InGaAs, AlGaAs, InGaAlAs, InGaP, InGaAsP, InGaAlP, InAlP or asequence of layers made of one or more of these materials.

Further advantageous refinements, features and details of the inventionemerge from the dependent claims, the description of the exemplaryembodiments and the drawings.

The invention is explained in more detail below using exemplaryembodiments in conjunction with the drawings. Only the elementsessential for understanding the invention are illustrated in each casein the drawings, in which:

FIGS. 1-6 show diagrammatic sectional illustrations of differentembodiments of the invention;

FIG. 7 shows a diagrammatic sectional illustration of the layer sequenceof the semiconductor laser of FIG. 2, and

FIG. 8 shows a diagrammatic sectional illustration of a furtherexemplary embodiment.

FIG. 1 shows an embodiment of an optically pumped vertically emittingsemiconductor laser (VECSEL) according to the invention in adiagrammatic illustration. A heat sink 12, for instance an SiC layer 10μm thick, is arranged between the Bragg mirror 10 and the active layersequence 14.

The laser resonator is formed by the Bragg mirror 10 and the externalconcave mirror 20, by means of which some of the radiation 22circulating in the resonator is also coupled out.

The VECSEL is optically pumped in a manner known per se, for example aVECSEL emitting at 950 nm through focusing of the multimode beam of an808 nm standard high-power pump diode.

The pump power is absorbed in the active medium, as a result of which,besides the desired laser emission, heat also arises, in particularthrough nonradiative recombination of excited charge carriers. This heathas to flow away via the Bragg mirror 10 into an adjoining substrate oranother heat reservoir.

The cross-sectional area of the heat flow is greatly enlarged by theheat sink 12 compared with the cross-sectional area of the laserradiation 22 through lateral thermal diffusion. The heat flow thereforeflows over a larger area through the Bragg mirrors 10, as a result ofwhich the thermal resistance is correspondingly reduced.

Another embodiment is shown in FIG. 2, where the heat sink 12 isarranged on that side of the active layer sequence 14 which is remotefrom the Bragg mirror 10. In this case, the heat generated in the activelayer sequence 14 flows into the heat sink 12, where it propagatesthrough lateral thermal diffusion over a large area before it finallyflows away via the Bragg mirror 10 into the substrate (not shown). Inthis case, structuring of the Bragg mirror can also achieve a situationin which a large part of the heat flows into the substrate whilebypassing the mirror.

One possible layer sequence for the embodiment of FIG. 2 is illustratedin FIG. 7. A Bragg mirror, comprising 40 layer pairs of AlAs (referencesymbol 42) and AlGaAs (reference symbol 44) with an aluminum proportionof at least 60%, is grown epitaxially on a GaAs substrate 40. Each ofthe layers 42, 44 has a thickness of λ/4n, that is to say a thickness ofabout 50 nm at an emission wavelength of λ=650 nm.

The active layer sequence 14 is grown on the Bragg mirror, said activelayer sequence comprising two InGaP quantum wells 48, 52 having a widthof about 5 nm and InAlP barriers 46, 50, 54 having a thickness of λ/4n.In this case, the number of quantum wells may also be larger, forexample 6 or 10. An SiC layer having a thickness of 10 μm is applied, asa heat sink, to the active layer sequence 14 for the purpose of lateralheat spreading.

In the embodiment according to FIG. 3, the mirror 10 is formed by a goldlayer. The lower reflectivity of such a mirror can be compensated for bya Bragg mirror 16 of reduced periodicity arranged between active layersequence 14 and heat sink 12. Owing to the smaller number of layersequences, the thermal resistance of this Bragg mirror 16 is also lower,thereby achieving overall a better heat dissipation in comparison withconventional configurations.

FIG. 4 shows a further embodiment of a VECSEL, in which heat sinks 12and 12′ are arranged on both sides of the active layer sequence 14. Theadvantages described in connection with FIGS. 1 and 2 are therebycombined.

It is furthermore possible to arrange a plurality of sequencescomprising active layer sequence and heat sink one behind the other, asshown in FIG. 5, where two active layer sequences 14 and 14′ in eachcase adjoin heat sinks 12, 12′ and 12′, 12″, respectively, on bothsides.

Another optically pumped vertically emitting semiconductor laser isillustrated in FIG. 6. In this configuration, the internal mirror isreplaced by a second external mirror, so that a vertical resonator isformed by two external concave mirrors 36, 38, in which the laserradiation 30 circulates.

The active layer sequence 34 is arranged between two heat sinks 32, 32′,in each case BN layers having a thickness of 15 μm in the exemplaryembodiment.

In the exemplary embodiment illustrated in FIG. 8, an optically pumpablevertically emitting semiconductor laser structure 140 is monolithicallyintegrated together with an edge emitting laser structure 150 on acommon substrate 100. The edge emitting laser structure 150 representsthe pump source for the optically pumpable vertically emittingsemiconductor laser structure 140. The optically pumpable verticallyemitting semiconductor laser structure 140 and the edge emitting laserstructure 150 are applied by means of epitaxy on the substrate 100, itbeing possible to grow the edge emitting laser structure 150 before orafter the epitaxial fabrication of the structure of the opticallypumpable vertically emitting semiconductor laser 140 on the commonsubstrate 100. Situated on the laser structures is a thermallyconductive layer 110, which serves as heat sink or heat spreader,situated on which, in turn, is a heat sink 120 with a radiation window130 arranged above the optically pumpable vertically emittingsemiconductor laser structure 140. Situated between the opticallypumpable vertically emitting semiconductor laser structure 140 and thesubstrate 100 is a bragg mirror 160, which, together with a bragg mirror170 arranged downstream of the radiation window 130 as seen from theoptically pumpable vertically emitting semiconductor laser structure140, forms the laser resonator for the optically pumpable verticallyemitting semiconductor laser structure 140.

1. An optically pumped vertically emitting semiconductor laser having ahighly reflective reflector layer (10), a radiation-emitting activelayer sequence (14) arranged on the reflector layer, in which, duringoperation, the reflector layer (10), together with an external mirror(20), forms a laser resonator, wherein a heat sink (12) transparent tothe emitted radiation is arranged within the laser resonator (10, 20)and in thermal contact with the active layer sequence (14), said heatsink being formed from a material having a higher thermal conductivitythan the materials of the active layer sequence (14).
 2. The verticallyemitting semiconductor laser as claimed in claim 1, in which the heatsink (12) is formed from readily thermally conductive materials whichare transparent in the red and/or near infrared, preferably from SiC, BNor diamond.
 3. The vertically emitting semiconductor laser as claimed inclaim 1, in which the heat sink (12) is formed by a layer having athickness of at least 0.2 times the diameter of the active zone.
 4. Thevertically emitting semiconductor laser as claimed in claim 1, in whichthe heat sink (12) is arranged between the active layer sequence (14)and the reflector layer (10).
 5. The vertically emitting semiconductorlaser as claimed in claim 4, in which the heat sink (12) is in directcontact with the active layer sequence (14).
 6. The vertically emittingsemiconductor laser as claimed in claim 4, in which a partly reflectiveBragg reflector (16) is arranged between the heat sink (12) and theactive layer sequence (14).
 7. The vertically emitting semiconductorlaser as claimed in claim 4, in which the reflector layer (10) is formedby a distributed Bragg reflector (42, 44).
 8. The vertically emittingsemiconductor laser as claimed in claim 6, in which the reflector layer(10) is formed by a metal layer.
 9. The vertically emittingsemiconductor laser as claimed in claim 3, in which the heat sink (12)or a further heat sink (12′, 12″) is arranged on that side of the activelayer sequence (14) which is remote from the reflector layer (10). 10.An optically pumped vertically emitting semiconductor laser having aradiation-emitting active layer sequence (34) and two external mirrors(36, 38), which form a vertical resonator, wherein a heat sink (32, 32′)transparent to the emitted radiation is arranged within the verticalresonator (36, 38) and in thermal contact with the active layer sequence(34), said heat sink being formed from a material having a higherthermal conductivity than the materials of the active layer sequence(34).
 11. The vertically emitting semiconductor laser as claimed inclaim 10, in which the radiation-emitting active layer sequence (14; 34)comprises a GaAs- or InP-based semiconductor material, in particularInGaAs, AlGaAs, InGaAlAs, InGaP, InGaAsP, InGaAlP, InAlP or a sequenceof layers made of one or more of these materials.
 12. The opticallypumped vertically emitting semiconductor laser as claimed in claim 10,in which the pump source (150) or a plurality of pump sources is or arearranged beside the optically pumped vertically emitting semiconductorlaser (140) and is or are monolithically integrated together with thelatter on a common substrate (100).
 13. The vertically emittingsemiconductor laser as claimed in claim 12, in which the pump source(140) or the plurality of pump sources has or have at least one edgeemitting laser structure.
 14. A vertically emitting semiconductor laser,in which the edge emitting laser structure or a plurality of edgeemitting laser structures is or are grown by means of epitaxy before orafter the epitaxial fabrication of the structure (100) of the opticallypumped vertically emitting semiconductor laser (140) on the commonsubstrate (100).
 15. The vertically emitting semiconductor laser asclaimed in claim 1, in which the radiation-emitting active layersequence (14; 34) comprises a GaAs- or InP-based semiconductor material,in particular InGaAs, AlGaAs, InGaAlAs, InGaP, InGaAsP, InGaAlP, InAlPor a sequence of layers made of one or more of these materials.
 16. Theoptically pumped vertically emitting semiconductor laser as claimed inclaim 1, in which the pump source (150) or a plurality of pump sourcesis or are arranged beside the optically pumped vertically emittingsemiconductor laser (140) and is or are monolithically integratedtogether with the latter on a common substrate (100).
 17. The verticallyemitting semiconductor laser as claimed in claim 16, in which the pumpsource (140) or the plurality of pump sources has or have at least oneedge emitting laser structure.